1. Field of the Invention
The present invention relates to a wafer boat rotating apparatus for rotating a wafer boat holding wafers.
2. Description of the Related Art
Generally, when semiconductor wafers and liquid crystal wafers are produced, oxide films, metal films, single-crystal films, and so forth are deposited or impurity-diffused on wafer surfaces. As units for performing such treatments, heat-treating units such as an oxidizing unit, a CVD unit, an epitaxial unit, and a diffusing unit are known.
In such a heat-treating apparatus, for example six lots (namely, 150 wafers) which are arranged with an equal pitch are held by a wafer boat made of heat resisting material which can withstand high temperatures and does not give rise to impurities at high temperatures. An example of such a heat resisting material is quartz.
Normally, as heat-treating apparatuses, upright type heat-treating furnaces have been widely used. In this heat-treating furnace, a wafer boat is placed in an upright (vertical) position. The wafer boat is transferred using one of the following two systems. One system is referred to as a stand-along system. In this system, wafers transferred by an arm to a wafer boat which is placed in a upright position. This arm carries one or five wafers at a time. The other system is referred to as batch system. In the batch system, 25 wafers are transferred to a wafer boat which is placed in a flat position at a time. After all 150 wafers are transferred, the wafer boat is raised.
In the stand-alone system, one heat-treating furnace requires a corresponding transfer unit. On the other hand, in the batch system, one transfer unit can handle a plurality of (for example, four) heat-treating furnaces. Thus, from the viewpoints of space and cost reduction, the batch system is superior to the stand-alone system. Therefore, the batch system is increasingly used.
When the batch system is used, as described above, a wafer boat rotating apparatus which places in an upright position and a flat position the wafer boat containing wafers is required.
The wafer boat rotating apparatus is disposed between the heat-treating unit and a wafer transfer unit. The heat-treating unit consists of, for example, four heat-treating furnaces arranged in series. The wafer transfer unit has a transfer mechanism which transfers for example twenty five (25) wafers held in a carrier to a wafer boat which is placed in a flat position. This wafer transfer mechanism is disclosed in for example U.S. Pat. No. 5,131,799 which is shown in FIGS. 7 and 8 thereof. This wafer boat rotating apparatus has a boat holding portion with a pair of arms which hold both ends of a wafer boat. The boat holding portion is mounted on a rotating shaft of a vertical rotation drive mechanism which has a motor and so forth. The vertical rotation drive mechanism rotates the boat holding portion on a vertical plane. The vertical rotation drive mechanism is mounted on a vertical drive mechanism which comprises a ball screw and a motor. The ball screw extends vertically. The motor rotates the ball screw. The vertical drive mechanism vertically moves the boat holding portion. A horizontal rotation drive mechanism is housed in a base and disposed at a lower end portion of the vertical drive mechanism. The horizontal rotation drive mechanism has a motor and so forth. As the vertical drive mechanism rotates on a horizontal plane, the boat holding portion accordingly rotates on the horizontal plane.
When wafers are transferred, a reception table of the transfer mechanism is raised from a downward position of the carrier on the wafer transfer unit, 25 wafers are held upward of the wafer transfer unit. The wafers held at the upper position are nipped by a wafer nipping member of the transfer mechanism. Then, the wafer nipping member is horizontally moved to the position above the wafer boat horizontally supported on a boat table of the wafer transfer unit. A reception table is raised from the downward portion of the wafer boat, and then the 25 wafers are supported by the reception table. The wafer nipping member is opened and then the reception table is lowered. In this way, the 25 wafers are transferred into the wafer boat. This operation is repeated for example four times. Thus, a total of 100 wafers are transferred.
After all the wafers have been transferred to the wafer boat, both ends thereof are held by the arms of the boat holding portion. Thereafter, the vertical rotation drive mechanism is rotated by 90.degree. so as to place the wafer boat in upright position. In addition, the horizontal rotation drive mechanism is rotated by 180.degree. so as to orient the wafer boat to the heat-treating unit.
By operating the vertical drive mechanism in a predetermined manner, the wafer boat is moved onto the transfer state of the heat-treating unit. This stage is moved to a transfer entrance/exit of a predetermined heat-treating furnace along a path connected thereto. The wafer boat placed on the stage is transferred to an elevator of the heat-treating furnace by a robot arm. The elevator vertically moves and the wafers held in the wafer boat are loaded into the heat-treating furnace.
When wafers which have been heat-treated are unloaded from the heat-treating furnace, the above-described operation is performed in reverse order.
The wafer boat comprises two holding plates and four wafer holding rods. Each of the wafer holding rods has a large number of (for example, 100) wafer holding grooves. By each groove, the peripheral portion of a wafer is held.
When wafers are transferred to the wafer boat which is placed in a flat position, a peripheral portion of each wafer comes in contact with a bottom portion of a corresponding wafer holding groove of the lower wafer holding rod. Thus, the wafer is held. In this condition, edge portions extruding from the holding plate are held and then the wafer boat is raised from the flat position to the upright position. Next, the wafers are transferred to the heat-treating furnance and heat-treated in the furnace.
However, there is a difference in coefficients of thermal expansion between quartz (which is a constructional element of the wafer boat) and a wafer. Thus, each wafer should be smooth against the inner wall surfaces of the wafer holding groove by the amount equivalent to the difference of the coefficients of thermal expansion. When each wafer is relatively light (as in a wafer with a diameter of six inches), the difference of coefficients of thermal expansion is absorbed without a problem. As the wafer size becomes large (as in eight inches), the weight of the wafer increases. For example, the weight of an eight-inch wafer is twice the weight of a six-inch wafer. Thus, when such large wafers are heat-treated, they do not satisfactorily slide and thereby the difference of coefficients of thermal expansion cannot be absorbed. Therefore, the stress which take place at the peripheral portion of each wafer becomes very large and thereby a slip dislocation takes place in the crystalline direction.
To solve such a problem, the shape of the wafer holding grooves may be changed in various manners. However, so far, the difference of coefficients of thermal expansion between wafer boat and wafers has not been satisfactorily absorbed.